With the proliferation of smartphones, mobile data traffic is growing tremendously. By taking notice of the increase of the number of smartphone users and the ensuing proliferation of diverse application services such as Social Network Service (SNS) and games, it is expected that the mobile data traffic is growing faster than ever before. Furthermore, if, beyond the human-to-human communication, the human-to-machine and machine-to-machine communications are popularized, this is likely to increase traffic concentration to the base station to the extent uncontrollable.
There is therefore a need of the technologies for solving the problems, and the Device to Device (D2D) communication is one of such technologies. The D2D communication is a technique promising in both the licensed band communication system such as cellular system and unlicensed band communication system such as Wireless Local Area Network (WLAN).
In the mobile communication field, the D2D communication draws attention particularly in terms of increasing the traffic accommodation capacity of the base station. The D2D communication is advantageous in that, since the User Equipment (UE) located within the same cell or adjacent cells can establish a D2D connection to communicate data directly without involvement of the evolved Node B (eNB), the number of communication links decreases from 2 (UE1-eNB and eNB-UE2 likes) to 1 (UE-UE link).
The LTE-based D2D communication includes a D2D discovery phase and a D2D communication phase. The D2D discovery is a procedure for a UE to check the identities and interests of the neighboring UEs and advertise its identity and interest to the neighboring UEs. The identity and interest can be identified by a UE identifier (ID), an application identifier, or a service identifier depending on the D2D service and operation scenario.
The protocol stack of the UE consists of a D2D application layer, a D2D management layer, and a D2D transport layer. The D2D application layer is responsible for D2D application-specific services running on the Operating System (OS), D2D management layer for converting the discovery information generated by the D2D application services to a format suitable for the D2D transport layer, and the D2D transport layer corresponds to the Physical/Media Access Control (PHY/MAC) layer of LTE or Wi-Fi radio communication standard. The D2D discovery procedure can be performed as follows. When a D2D application is executed, the application layer generates the discovery information to the D2D management layer. The management layer converts the discovery information to a management layer message. The management layer message is transmitted by means of the transport layer, and the neighbor UE receives and processes the management message in the reverse order of transmission.
As aforementioned, the goal of the D2D communication is for the UEs to communicate data directly without any infrastructure such as eNB and Access Point (AS). The D2D communication can be performed based on the result of the D2D discovery procedure (with other UEs) or without discovery procedure. Whether the D2D discover procedure is required depends on the D2D service and operation scenario.
The D2D service scenarios are classified into a commercial service (or non-public safety service) and a public safety service. Examples of the services include advertisement, Social Network Service (SNS), gams, and public safety and emergency network service.
1) Advertisement: The network operator supporting the D2D communication allows the preregistered shops, cafés, theater, restaurants, etc. to their identities to the D2D users near around through D2D discovery and D2D communication procedures. The interest includes advertiser's promotion, event information, and discount coupon. When the identity matches the user's interest, the user visits the corresponding shop to obtain detailed information through the legacy cellular communication network or the D2D communication. In another example, a user discovers the taxis located around the user through the D2D discovery procedure and exchange data about user's destination and taxi fare through the legacy cellular communication or D2D communication.
2) Social Network Service (SNS): The user transmits the current application and application-specific interests to other users located near the user. The identity and interest for use in the D2D discovery includes application-specific friend list and application identifier. The user performs D2D discovery and then share the contents such as photos and motion pictures through the D2D communication.
3) Game: The user discovers the users and game applications to do a mobile game with other near users through the D2D discovery procedure and perform D2D communication for exchange game data.
4) Public safety service: The police and firefighters use the D2D communication technology for public security purpose. For example, in an emergency situation such as a fire, a landslide, or when the cellular communication is cut off due to the cellular network breakage caused by a natural disaster such as earthquake, volcano eruption, and tsunami, the police and firefighters use the D2D communication to discover the near colleagues and share emergency situation information with the near users.
The 3 GPP LTE D2D standardization is progressing in both the D2D discovery and D2D communication but different in standardization range. Both the D2D discovery and D2D communication are developed for the commercial use and have to be designed only in the network coverage. The D2D discovery does not support non-eNB environment or out of the eNB coverage. The D2D communication is developed for the public safety and emergency network service other than commercial service and has to support all the situations, such as in and out of network coverage and in the partial network coverage (communication in a situation where some UEs are located in the eNB coverage and other UEs out of the eNB coverage). In the public safety and emergency network service, it is required to perform the D2D communication without D2D discovery procedure.
Both the D2D discovery and D2D communication of the LTE D2D under the way of standardization are implemented in association with LTE uplink subframe. That is, a D2D transmitter transmits a D2D discovery signal and D2D communication data in the uplink subframe, and the D2D receiver receives the signal and data in the uplink subframe. In the current LTE system, since the UE receives data and control information from the eNB in downlink and transmits data and control information to the eNB in uplink, the D2D operation differs from the legacy LTE. For example, a UE that does not support D2D function has to have a receiver implemented based on Orthogonal Frequency Division Multiplexing (OFDM) for receiving downlink data and control information from the eNB and a transmitter implemented based on Single Carrier—Frequency Division Multiplexing (SC-FDM) for transmitting uplink data and control information to the eNB. However, the D2D UE has to have an SC-FDM receiver for receiving the D2D data and control information in uplink as well as the OFDM-based receiver for receiving downlink signal from the eNB and a SC-FDM transmitter for transmitting uplink data and control information to the eNB and D2D data and control information to the peer D2D UE because it has to support both the cellular mode and D2D mode.
The current LTE D2D specifies two types of D2D discovery schemes that are used selectively depending on the resource allocation method.
1) Type 1 discovery: The eNB broadcasts the information on the uplink resource pool available for D2D discovery through a System Information Block (SIB) in order for the UEs within the cell to receive. The size of the resource available for D2D communication (such as x consecutive subframes) and the resource periodicity (such as y seconds) are informed. When this information is received, the receiving D2D UEs select the resources to use in a distributed manner and transmit D2D discovery signals using the selected resources. Meanwhile, the receiving D2D devices have to receive all the D2D discovery signals transmitted in the resource pool indicated by the SIB.
2) Type 2 discovery: The eNB notifies the receiving D2D UEs of the resource pool for the discovery signals through the SIB. The discovery signal resource for the sending D2D UEs is scheduled by the eNB. At this time, the eNB performs the scheduling in a semi-persistent manner or a dynamic manner.
Like the D2D discovery procedure, the D2D communication procedure can be classified into two modes depending on the resource allocation type:
1) Mode 1: The base station notifies of the data transmission resource for D2D communication which the D2D transmitter used directly.
2) Mode 2: the eNB notifies of the resource pool which the D2D transmitter can use such that the UEs selects the resources in a distributed manner to transmit signal.
Another characteristic of the LTE-based D2D communication is that it aims the public safety scenario, unlike the cellular communication focused on unicast communication, to support broadcast-based communication. Accordingly, the LTE D2D communication does not support feedback such as channel measurement report and Hybrid Automatic Repeat Request Acknowledgement/Negative-acknowledgement (HARQ ACK/NACK). From this viewpoint, the pending question of the D2D broadcast communication is to providing highly reliable link quality for guaranteeing seamless D2D communication without assistance of eNB and any feedback. Particularly in the scenario where the UEs operate in a distributed manner with no assistance of eNB, it is important to solve to solve the resource collision problem caused by contention among the UEs for occupying the same resource.
As described above, since the D2D communication for the public safety network has to operate even in the situation without assistance of any eNB and the UEs participated in the D2D communication transmit no feedback, there is a need of a method capable of controlling the D2D resource efficiently among the UEs.
There are a few well-known resource allocation methods for D2D communication as follows.
The legacy Wi-Fi or ZIGBEE-based ad-hoc/sensor network uses Carrier Sense Multiple Access/Collision Avoid (CSMA-CA) as the basic contention-based resource access scheme without the concept of scheduling-based resource allocation. The CSMA-CA is characterized in that, when the number of UEs is small, the communication is performed in a way of suspending transmission to avoid collision without extra complex network management and thus used widely. However, it shows a drawback of significant data rate drop in the area where Wi-Fi users are concentrated so as to increase the user complaints and thus there is a need of enhanced method capable of substituting for the CSMA-CA.
Unlike the above-described D2D distributed resource access methods, the Time Division Multiple Access (TDMA) is the most efficient resource access scheme when a master node manages the resource. In the situation where a plurality of master nodes exist, however, there is a need of controlling resource allocation among the master nodes and this causes extra control signal overhead and delay. As a consequence, the TDMA is not suitable for the network expandable to cover a wide area such as D2D communication network.
FlashLinQ of QUALCOMM modifies the Request To Send (RTS) and Clear To Send (CTS) controls signals used in the CSMA-CA for access to the TCMA resource. It is known that the FlashLinQ shows performance increase as much as 5-fold compared to the Wi-Fi in a specific environment when applying the Signal-to-Interference Ratio (SIR) measured using the RTS and CTS to the terminal operating based on OFDM in the synchronized network.
Among the conventional technologies, the contention-based method such as CSMA-CA shows good extensibility but low efficiency, and the resource access method such as TDMA shows high efficiency but poor extensibility. The FlashLinQ, which has been conceived to solve these problems, introduces the slot concept as in the TDMA for efficiency and uses the round robin scheme for allocating slot resources.
The D2D broadcast communication for public safety network which is under discussion in the 3GPP is similar to Wi-Fi in that there is no need of connection setup and to FlashLinQ in that it operates on the licensed band and establishes synchronization between terminals.
There is therefore a need of a method for allocating D2D resource efficiently in the network area with assistance of base station while minimizing the collision probability in contention-based resource allocation in consideration of the D2D communication (or D2D broadcast) characteristics.